Important Short Questions and Answers: Conventional and Solid State Speed Control of D.C Drives

Conventional & Solid State Speed Control of DC Drives

Give the expression for speed for a DC motor?

Speed N = k (V-I_aR_a) /  j

Where V = Terminal Voltage in volts

Ia = Armature current in Amps

Ra = Armature resistance in ohms j= flux per pole.

What are the ways of speed control in dc motors?

Field control (i) by varying the flux per pole.-for above rated speed

(ii)In armature control method the speed below base speed is obtained.

Armature control (i) by varying the terminal voltage -for below rated speed

(ii)In field control method the speeds above base speed can be obtained.

Give the Limitation of field control?

Speed lower than the rated speed cannot be obtained.

It can cope with constant kW drives only.

This control is not suitable to application needing speed reversal.

What are the 3 ways of field control in DC series motor?

Field diverter control

Armature diverter control

Motor diverter control

Field coil taps control

Series-parallel control

What are the main applications of Ward-Leonard system?

It is used for colliery winders.

Electric excavators

In elevators

Main drives in steel mills and blooming and paper mills.

What are the merits and demerits of rheostat control method?

Impossible to keep the speed constant on rapidly changing loads.

A large amount of power is wasted in the controller resistance.

Loss of power is directly proportional to the reduction in speed. Hence efficiency is decreased.

Maximum power developed is diminished in the same ratio as speed.

It needs expensive arrangements for dissipation of heat produced in the controller resistance.

It gives speed below normal, not above.

What are the advantages of field control method?

More economical, more efficient and convenient.

It can give speeds above normal speed.

Compare the values of speed and torque in case of motors when in parallel and in series.

The speed is one fourth the speed of the motor when in parallel.

The torque is four times that produced by the motor when in parallel.

Mention the speed control method employed in electric traction. (UQ)

Series-parallel speed control.

What is the effect of inserting resistance in the field circuit of a dc shunt motor on its speed and torque?

For a constant supply voltage, flux will decrease, speed will increase and torque will increase.

While controlling the speed of a dc shunt motor what should be done to achieve a constant torque drive?

Applied voltage should be maintained constant so as to maintain field strength.

What are the advantages of ward –Leonard Scheme?

 Wide range of speed variation is possible

 Regenerative action is possible          

 Good speed regulation             

 Stepless and smooth speed control in either direction is possible.

What are the disadvantages of ward –Leonard Scheme?

 Overall efficiency of the system is low especially at light loads

 Costlier since two extra machines are required

Differentiate controllable and uncontrollable rectifiers

Controlled rectifiers  

Use only diodes         

Output voltage is variable

Uncontrolled rectifiers        

Use only thyristors   

Output voltage is fixed       

Define holding current in SCR?

Holding current of SCR is defined as the maximum forward current that must be maintained to keep the SCR in the conducting state. If the current through the SCR is reduced below the level of holding current, the device returns off.

Define chopper?

A chopper is dc to dc converter. The fixed voltage of a dc source can be converted into an adjustable average voltage across a load by inserting a high speed switch between the dc source and the load .This high speed switch is called the chopper.

What is time ratio control?

Varying output voltage by varying time period of duty cycle is called as Time ratio control.

Write the methods of obtaining time ratio control?

By varying the duration of ON time with respect to off time keeping total time period T constant

By keeping ON time constant and varying the frequency.

By adjustment of both

How to classify rectifier circuits? Define phase controlled rectifier? And write the applications

Uncontrolled rectifiers

Controlled rectifiers

Controlled rectifier converts fixed ac voltage to variable dc output voltage.

Dc traction

Magnet power supplies

High voltage dc transmission

Electrochemical and electrometallurgical process

Portable hand tool drives

What is meant by step -up and step -down chopper?

The average output voltage Vo is less than the input voltage Vs (Vo<Vs) - step-up chopper or buck converter

The average output voltage Vo is greater than the input voltage Vs (Vo>Vs) - step-up chopper or boost converter

Define duty cycle?

Duty cycle is defined as the ratio of the on time of the chopper to total time of the chopper.

Write the application and advantages of chopper? Applications

Battery operated vehicles

Traction motors control in electric traction

Trolley cars

Electric braking

Marine hoists

Mine haulers

Advantages

Chopper control provides

High efficiency

Smooth acceleration

Fast dynamic response

Regeneration

What are the different types of commutation in chopper? Define it.

Voltage commutation

When the charge capacitor momentarily reverses biases the conducting thyristor and it turn off.

Voltage commutation

When a current pulse is made to flow in the reverse direction through the conducting thyristor current is zero, it is turned off.

Load commutation

When the load current flowing through the thyristor either becomes zero or is transferred to another device from the conducting thyristor.

What are the different types of controlled rectifier? According to input supply

Single phase controlled rectifier

Three phase controlled rectifier

According to the quadrant operation

Semi converter

Full converter

Dual converter

According to number of pulse per cycle

One pulse converter

Single phase half controlled rectifier

Two pulse converter

Single phase half controlled rectifier

Single phase full controlled converter

Three pulse converter

Three phase half wave controlled rectifier

Three phase full controlled converter

Six pulse converter

Three phase full converter

Twelve pulse converter

Define half controlled rectifier, full converter?

A half controlled rectifier or semi converter uses a mixture of diodes and thyristors and there is a limited control over the level of dc output voltage. it is also known as one-quadrant converter.

A fully controlled converter or full converter uses thyristors only and there is a wider control over the level of dc output voltage .it is also known as quadrant converter.

What is the function of free-wheeling diode in controlled rectifier? And write its advantages?

The freewheeling diode is used to circulate the current within the motor armature circuit itself. The fee-wheeling diodes considerably improve the rectifier power factor and the motor performance.

Advantages

Input factor is improved

Load current waveform is improved and thus the load performance is better.

What is the control techniques used in chopper controlled drives?

Time ratio control (TRC).

Current limit control (CLC).

Explain in brief about Time ratio control?

The TRC, also known as pulse-width control, the ratio of „ON‟ time to chopper period is controlled. The TRC is again divided into two types:

Constant frequency TRC:

The chopping period „T‟ is kept constant and varying the „ON‟ period of the switch S, the duty ratio d is controlled.

Variable Frequency TRC:

In this the duty ratio d is varied either by keeping t_ON constant and varying T or by varying both t_ON and T. This method is rarely used because of the following drawbacks.

Low output voltage at low chopper frequencies.

Low frequency operation adversely affects the motor performance.

Design of input filter becomes difficult.

What are the advantages of chopper drives over rectifier drives?

Advantages:

This however reduces the torque per ampere at zero and low speeds.

There is lot of problems with re-generative braking of a series motor.

On the other hand, re-generative braking of a separately excited motor is fairly simple and can be implemented down to very low speeds.

Because of the limitations of a D.C. motor, separately excited motors are preferred even for traction at present.

What are the advantages in using chopper for speed control of DC motors?

Advantages:

High Efficiency

Flexibility in control

Light weight

Small size

Quick response

Re-generation down to very low speed.

What are the applications of DC drives?

The applications of DC drive are:

Steel rolling mills

Paper mills

Printing press

Mine winders

Machine tools

Write some special features of thyristor drive motors?

The use of octoganal,rather than circular, shape for the frame accommodates more material and gives a larger rating for the same frame size

The yoke as well as the main and commutating poles are laminated to reduce the eddy current effects. the use of a laminated yoke instead of a solid yoke improves the commutation to a greater extent

The thyristors drive dc motors are made with larger diameter armature and larger size poles of reduced height.

Low inertia armatures are employed for improving the response

Compensating windings are used in large motors to reduce for reducing the effect of transformer voltage in the coils undergoing commutation.

The use of large number of commutator bars reduces the voltage between commutator segments and improves the commutation.